One of the better books Gerry Spence has written was his book Win Your Case. His advise is on target. here are a few points he makes in the book.

• Avoid      sarcasm, scorn, and ridicule. Use humor cautiously. Hold      back insult. No one admires the cynic, the scoffer, the mocker, the small,      and the petty. Giving respect to one’s opponent elevates us. Those who      insult and slight do so from low places. Remember: Respect is reciprocal.      The employment of humor can be the most devastating of all weapons in an      argument. Humor is omnipotent when it reveals the truth. But beware:      attempting to be funny and failing is one of the most dangerous of all      strategies.
• Logic      is power.      If      logic is on your side, ride it–ride it all the way. If logic is not on      your side, if logic leads to an unjust result, it will have no power. As      Samuel Butler said, “Logic is like the sword–those who appeal to it      shall perish by it.” Logic does not always lead to truth or justice.      Logic defeats spontaneity. Logic is often dull and is more comfortable      with the dead, for it is often without spirit. Do not give up creativity      for logic. However, the creative mind will soon see that creativity is      often served by logic.
• Action      and winning are brothers. The      worst of head-on attacks is often better than the most sophisticated      defense. Never permit your opponent to take control. Do not defend when      you can attack. Counter punching is for boxers, and counterpunchers most      often lose. The great champions of the world take control. The great      generals attack first, and attack again. Take the initiative. Do something.      But with those we love, the best attack is often to attack with love, and,      as we shall see, winning is often accomplished by the art of losing.
• Admit      at the outset the weak points in your argument. You      can expose your weaknesses in a better light than your opponent, who will      expose them in the darkest possible way. An honest admission, having come      from you, not only endows you with credibility, it also leaves your      opponent with nothing to say except what you have already admitted.
• Understand      your power. Give yourself permission–only to win.      But      remember, arrogance, insolence, and stupidity are close relatives.